The Oath took Maedhros, his father and his brothers to Middle-earth during the First Age where they established realms in exile, waged war against the armies of Morgoth, fought their own Elvish kind, and eventually brought ruin upon their House.

Shortly after the Dagor-nuin-Giliath in which Fëanor was killed, Maedhros was captured by Morgoth and hung by the wrist of his right hand upon the face of a precipice of Thangorodrim. In a daring rescue, his cousin Fingon, helped by Thorondor the King of Eagles, saved him from torment but he had to cut off Maedhros's hand to release him from the shackle. In gratitude for this, and in atonement for Feanor's desertion of Fingon's house, Maedhros relenquished all claim as the heir of Finwë and made his uncle Fingolfin, Fingon's father, High King of the Noldor, something his brothers did not like.

Seeing that his brothers were likely to cause feuds with their kinsmen, Maedhros moved them out of Hithlum, and later ruled the lands around the Hill of Himring, which became known as the March of Maedhros. Allied with Fingolfin, he won the battle of Dagor Aglareb, and, thanks to his daring deeds during the Dagor Bragollach, Himring stood while many other elven realms fell.

Hearing words of deeds of Beren and Lúthien, he took hope and gathered his brothers and united with other Elven Houses to create the Union of Maedhros, an alliance to lay siege to Morgoth's fortress of Angband. The Union and the siege was utterly broken after the defeat in the Nírnaeth Arnoediad. Himring was garrisoned by Orcs.

Maedhros had learned that Elwing of Doriath, granddaughter of Lúthien and Beren, had inherited the Silmaril that they had recovered from Morgoth. Still driven by the Oath, he allowed his brother Celegorm to convince him to attack Doriath. Celegorm, Caranthir, and Curufin were slain by Beren's son, Dior Eluchíl, the King of Doriath, who was in-turn slain by them. Dior's sons, Eluréd and Elurín, were captured and abandoned by Celegorm's servants in the forest, leading Maedhros to search for the innocent youths. After learning that Elwing had survived, he and his surviving brothers descended with an army upon the remnants of the people of Doriath living in the Havens of Sirion. The Noldorin princes killed many Elves and captured Elwing's sons Elrond and Elros in the sack, but she and her husband Eärendil escaped to the West with the jewel.

After the War of Wrath, he and his last surviving brother, Maglor stole the two remaining Silmarils taken by the Valar from Morgoth. But because of the evil deeds committed by the brothers to regain the jewels, they burned in Maglor's and Maedhros's hands. Unable to bear the suffering, Maedhros cast himself and the Silmaril he carried into a fiery chasm in the Earth.

Etymology

Maedhros' father-name was Nelyafinwë, meaning "Finwë the Third", shorter form Nelyo, as his grandfather and father both shared the name Finwë. Nelya is a Quenya adjective that means "third". His mother-name was Maitimo, "Well-shaped One", for he was noted for his comeliness. His epessë was Russandol, "Copper-top", referring to the dark red hair he inherited from his grandfather Mahtan. It is derived from russa, a Quenya adjective meaning "red-haired". The name Maedhros is Sindarin, and is said to mean "Pale-glitter" in The Etymologies. It is formed by the adding up the "Noldorin" maidh, meaning "pale", "fallow" or "fawn" and archaic "Noldorin" form rhoss, which translates as "flash", "glitter of metal". However, in The Shibboleth of Fëanor, note# 65, the Sindarin form Maedhros is explained as having been derived from a contraction of his mother-name with his epessë (Mait- + Rus-).